1. Field of the Invention
The present invention relates generally to time of day clock systems employing crystal oscillators for providing reference frequencies from which time periods are derived, and more particularly, to improved time of day clocks and crystal oscillator control techniques therefor which minimize errors in adjusting the output frequency of the oscillators.
2. Description of the Prior Art
In prior art time-of-day clock systems, it is wellknown to use a crystal input to control the internal oscillator of a microcomputer. The internal oscillator in most microcomputers is designed to interface with a parallel resonant quartz crystal of a specified frequency. A crystal pi network is connected between the internal oscillator and the crystal. The pi network usually has a variable capacitor which is used to vary the tuning range of the oscillator.
To accurately adjust the oscillator's output frequency to a desired rate for use in the microcomputer (e.g., 8 MHz), the variable capacitor is used as a trimmer capacitor to vary the frequency range of the oscillator while a suitable frequency calibration meter is used to monitor the changes in frequency as the anti-resonance of the crystal is changed. Once the trimming of the oscillator is completed, usually a sealant is used to affix the trimmer capacitor's sealant in place. But sometimes, the very act of affixing the sealant causes the adjustment to move from its optimum position. Once the capacitors are sealed and the oscillator is properly trimmed, the accuracy of the oscillator may still be affected because typical trimmer capacitors usually are susceptible to variations in environmental conditions such as humidity and temperature.
Another technique for adjusting the frequenc:y output of the oscillator is to use digital correction techniques such as those described in U.S. Pat. No. 4,282,595, by Lowdenslager et al. entitled "Method For Digital Frequency Trimming an Oscillator in an Electronic Timepiece." There, the effective frequency of an oscillator in a watch is adjusted by periodically inhibiting pulses to a divider stage. A pulse inhibit circuit includes a nonvolatile programmable ROM for storing binary complement information corresponding to the number of oscillator pulses needed to be suppressed to achieve the effective frequency. A counter is periodically preset with the binary complement information and the count advances in response to the oscillator's pulses. The difference count between the complement number and the counter's maximum count controls the number of pulses periodically suppressed. During the manufacture and assembly of the watch, the ROM is programmed to contain the desired complement information.
This technique does permit effective trimming of the oscillator's frequency; however, the correction procedures can only make the oscillator slower. There must be a guarantee that the uncorrected oscillator frequency is of a rate greater than the effective frequency needed for the watch.
In motor vehicles equipped with digital computers, along with vehicle electronics and instrumentation which provides visual displays of a variety of information related to the condition and operation of the motor vehicle, a time-of-day clock is usually provided. The time-of-day clock must be capable of displaying minutes, hours, days of the week, days of the month and months accurately to at least 80 seconds per month (i.e., 0.003%). This is a fairly loose specification due to the extreme environmental conditions the time-of-day clock experiences in an automobile. However, to meet at least this specification, any software programming needed to program the system's computers should not contribute to timing inaccuracies.
Crystals used in crystal oscillators typically having an accuracy of about .+-.0.01% which, if not adjusted, will result in errors of about nine seconds a day and nearly five minutes a month which is too much error to meet the above-mentioned loose specification.
Hence, in developing a time-of-day clock with improved accuracy, it is desirable to minimize errors when adjusting the output frequency of the oscillator in the microcomputer being used. It is also desirable to perform the adjusting with automated equipment, without sacrificing effectiveness and to employ as few additional hardware components as necessary. Also, it is desirable that the timing circuitry is not susceptible to environmental conditions such as humidity and temperature and to mechanical vibrations of the motor vehicle.
To obtain the above-mentioned desiderata, a search for various other ways to calibrate crystal oscillators was initiated. This search resulted in the improved apparatus and techniques of the present invention.